Hitherto, as recording media for information signals such as sound signals and/or video signals, and data, etc. processed at information processing equipment such as computer, etc., various kinds of optical discs have been used and have been proposed.
As the optical disc of this kind, there are used optical discs adapted for recording sound signals and/or data, etc. processed at information processing equipment such as computer, etc., and having diameter caused to be 120 mm or 80 mm and thickness caused to be 1.2 mm. Such optical discs are called Compact Disc (CD).
Moreover, there are also used video discs which are optical discs adapted for mainly recording video signals as information signal, and having diameter caused to be 300 mm or 200 mm and thickness caused to be 1.2 mm.
Further, with realization of multi-media of information signals, there has been demand for realization of wide variety and vast (gigantic) quantity of information signals such as picture data or sound data, etc. which are handled at a time. To satisfy such demand, as optical disc in which the medium itself has been miniaturized while realizing high recording density, there have been proposed optical discs in which two optical discs each having diameter caused to be 120 mm and thickness of the disc base (substrate) caused to be 0.6 mm are stuck to each other so that the entire thickness is 1.2 mm, and optical discs in which optical disc having thickness of 0.6 mm and disc reinforced plate having thickness of 0.6 mm are stuck to each other so that the entire thickness is caused to be 1.2 mm. Such optical discs are generally called Digital Video Disc (DVD).
In general, the optical disc constituted as a recording medium for information signals are adapted as shown in FIG. 1 so that non-signal recording areas 202, 203 are respectively provided at the inner circumferential side area of the outer periphery of a center hole 201 formed at the central portion and the outermost circumferential side area, and a signal recording area 204 where information signals are recorded is provided within the area encompassed by the inner circumferential side non-signal recording area 202 and the outer circumferential side non-signal recording area 203. At the inner circumferential side of the signal recording area 204, there is provided a TOC (Table of Contents) area 205 in which addresses of information signals recorded in the signal recording area and management information such as table (of contents) of recording contents, etc. are recorded. Further, in the optical disc, in the state where the innermost circumferential side position of the TOC area is caused to be read starting position of information signals, i.e., lead-in position, read operation of information signals recorded on the optical disc is started from the lead-in position. In addition, in CD 210 having diameter caused to be 120 mm, the lead-in position is set at the position where radius R.sub.1 from the disc center O.sub.1 is 23 mm.
In order to carry out read operation of information signals recorded on this optical disc, the optical pick-up device is caused to be moved toward the innermost circumferential side position of the optical disc. At this time, the object lens (objective) which focuses light beams emitted from the light source of the optical pick-up device to irradiate them onto the optical disc is adapted so that its optical axis is caused to be in correspondence with the lead-in position. In addition, when the lead-in position which serves as reference position of read starting of information signals is caused to undergo movement operation toward the outer circumferential side of the optical disc, the optical pick-up device scans the signal recording area of the optical disc by light beams emitted from the light source to carry out read operation of information signals.
Meanwhile, in the recording and/or reproducing apparatuses for optical disc using optical disc as recording medium, there have been proposed apparatuses adapted for reproducing, by using the same optical pick-up device, information signals recorded on optical discs of plural specifications in which their lead-in positions are different from each other.
As the recording and/or reproducing apparatus for optical disc of this kind, there are used optical disc recording and/or reproducing apparatuses capable of reproducing, with compatibility, CDs and video discs in which their lead-in positions are different from each other. In such optical disc recording and/or reproducing apparatuses, attention is drawn to the fact that diameters of the CD and the video disc are different from each other to discriminate sizes of these optical discs by the discrimination mechanism to control the lead-in position of the optical pick-up device in dependency upon its discrimination result to carry out reproduction of information signals from lead-in positions corresponding to respective optical discs.
In recording and/or reproducing apparatuses adapted to adjust (change) the lead-in position of the optical pick-up device in correspondence with the specification of the optical disc used as stated above, the movement range of the optical pick-up device is caused to be broad as compared to the size of the optical disc used. As a result, not only the apparatus itself becomes large-sized, but also feed control of the optical pick-up device becomes complicated because the reference position of the optical pick-up device is changed.
Additionally, also in CD and DVD, lead-in positions of the optical pick-up device are different from each other.
Moreover, in the recording and/or reproducing apparatuses for optical disc using optical disc as recording medium, there have been proposed apparatuses capable of using, with compatibility, optical discs in which thicknesses of their disc substrates are different from each other, e.g., CD and DVD, and CD, DVD and video disc. When the thickness of the optical disc is different, the distance from the object lens up to the signal recording surface of the optical disc would be varied. In order to use, with compatibility, optical discs of plural specifications in which their thicknesses are different from each other, plural object lenses (objectives) having focal lengths (distances) different from each other are required.
In optical pick-up devices provided with plural object lenses, plural object lens are caused to undergo rotational operation so that every time each of optical discs different in thickness is loaded, the optical axis of object lense corresponding to each optical disc is positioned on the center line of the loaded optical disc. However, when the mechanism for allowing the object lenses to undergo rotational operation is provided, the optical pick-up device becomes large-sized. When the optical pick-up device becomes large-sized, the feed mechanism of the optical pick-up device also becomes large-sized. As a result, the optical disc recording and/or reproducing apparatus itself also becomes large-sized.
Moreover, in optical pick-up devices used in optical disc recording and/or reproducing apparatuses capable of using, with compatibility, optical discs in which thicknesses of their disc bases are different from each other, there have been also proposed optical pick-up devices in which single object lens is used to adjust (vary), in dependency upon thickness of the optical disc to be loaded, distance of the object lens with respect to the optical disc, i.e., the reference height position with respect to the optical disc of the object lens. However, in order to change the reference height position of the object lens, the mechanism for allowing the object lens to undergo adjustable driving in the optical axis direction thereof becomes complicated. In addition, the mechanism for displacably supporting the object lens in the optical axis direction thereof becomes complicated.